1. Field of the Invention
The present invention relates to a process for the pyrolysis of urethanes to the corresponding isocyanates. The process allows the production of mono-, di-, and polyisocyanates while eliminating problems associated with prior processes.
2. Description of the Prior Art
The industrial production of organic mono- and polyisocyanates is generally performed by the phosgenation of the corresponding organic amines to carbamic acid chlorides followed by pyrolysis. This process, however, is subject to well known disadvantages with respect to environmental protection and safety due to the toxic nature of both starting materials and process by-products.
Isocyanates may also be produced by subjecting to pyrolysis N-substituted urethanes either in the gaseous or in the liquid phase. However, various additional undesired side reactions frequently take place during such thermal cleavage. Typical examples of these side reactions are decarboxylation of the urethanes, which may be accompanied by the formation of primary and secondary amines as well as olefins; the reaction between the isocyanate and urethane which produce allophanates and amine which in turn may react to form ureas; and the polymerization of the isocyanates to form crystalline isocyanurates.
According to German patent document No. 19 44 719 (British Patent No. 1,247,451), for example, urethanes are pyrolyzed at temperatures from 400.degree. to 600.degree. C. in the presence of Lewis acid catalysts. The isocyanate and the alcohol produced by the pyrolysis are separated by means of fractional condensation. By this process, for example, 2,4-toluene diisocyanate may be obtained through the pyrolysis of toluene-2,4-diethylurethane in the presence of iron(III) fluoride. Some of the disadvantages of this reaction are low yields, significant amounts of polymeric by-product, decomposition of the catalyst, and corrosion of the reaction equipment. In U.S. Pat. No. 3,870,739 a process is described in which an aromatic urethane is subjected to pyrolysis at a temperature from 350.degree. to 550.degree. C. and a pressure less than (m+1) times the isocyanate vapor pressure wherein m is the number of urethane groups in the aromatic urethanes. The pyrolysis occurs in a catalyst-free pyrolysis zone wherein the residence time of the pyrolysis products is less than 15 seconds. One of the disadvantages of this process is that a solid polymer is produced as a by-product. This solid polymeric by-product clogs the pyrolysis reactor and associated equipment necessitating both extensive separation problems as well as frequent process down time. These problems make a continuous process difficult to operate.
Thermal cleavage of urethanes in the liquid phase is described in U.S. Pat. No. 3,962,302 and U.S. Pat. No. 3,912,280. In the former, the urethanes are dissolved in an inert solvent such as alkylbenzenes, aliphatic and cycloaliphatic hydrocarbons, or phthalate esters and are cleaved under normal or superatmospheric pyrolysis reactor pressure at temperatures of from 175.degree. to 350.degree. C. The isolation and separation of the resulting isocyanate and alcohol are achieved either by using the solvent or an inert gas as a carrier. In U.S. Pat. No. 3,912,280, high molecular weight substituted or unsubstituted aliphatic, cycloaliphatic, or aromatic hydrocarbons, ethers, esters, or ketones are used as the solvent. Separation of the cleavage products is achieved by distillation. Isocyanate, alcohol, and inert gas carrier are taken off as the overhead, while the solvent is taken off as the bottom fraction.
In order to produce aromatic isocyanates by the process of U.S. Pat. No. 4,081,472, urethanes are pyrolyzed at temperatures of 150.degree. to 350.degree. C. under reduced pressure through contact with a solution containing at least 0.001 weight percent of at least one metal ion, such as ions of copper, zinc, aluminum, tin, titanium, vanadium, iron, cobalt, and nickel as catalyst, dissolved in a solvent having a boiling point of 200.degree. C. The separation of the resulting cleavage products is achieved by means of fractional condensation. Here, however, polymerization products which cannot be distilled off remain in the solvent fraction which means that additional purification operations are required for the solvent containing the catalytically active metal ions.
In U.S. Pat. No. 4,330,479 pyrolysis is achieved by utilizing catalytically active, large-surface area metals present in a heterogenous phase. The disadvantage of this process is that the metals used as catalysts gradually lose their catalytic activity over time as a result of reduced surface availability caused by surface coating which consists of polymeric reaction by-products. Removal of these by-products also necessitates additional purification operations.
In order to eliminate this problem, European patent document No. 35 519 performs thermal cleavage in the presence of carbon, preferably as a solids charge in a fluidized bed. The advantage here is that the catalyst, deactivated by the polymeric by-products or decomposition products, does not need to be regenerated, but rather can be destroyed in a residue-free and environmentally sound manner by means of incineration. However, certain technical expenses are unavoidably linked to the sophisticated fluidized bed technology and to the regeneration procedures required for the fluidized packing.